1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and in particular, to a method for flattening a surface after film formation. The present invention also relates to a polishing apparatus for such flattening.
2. Description of the Related Art
As semiconductor integrated circuits become more and more integrated, flattening technology for uniformly flattening a surface of a substrate in order to form microscopic patterns with high precision are becoming more important during the manufacturing process. As one of such conventional flattening technologies, there is widely used a CMP (chemical mechanical polishing) method in which a substrate is pressed against a polishing cloth (pad) for polishing with use of an abrasive liquid (slurry).
In cases where particularly high flatness is required at the time of flattening in accordance with the CMP method, namely, in such cases where an extra portion of an insulating film with which a trench is filled in is removed by polishing, the trench being created in an STI (shallow trench isolation) method upon forming an element isolation region in accordance with the STI method, a slurry having abrasive grains of ceric oxide (also referred to as “ceria”) is widely used, as described in Japanese Unexamined Patent Publication No. 2001-310256.
This is because a higher polishing rate for silicon oxide films and a higher polishing rate selectivity for silicon nitride films used as a polishing stopper film can be obtained upon using a slurry having abrasive grains of ceric oxide and an appropriate organic compound as an additive, in comparison with cases of using a slurry having abrasive grains of silicon oxide (silica), which is conventionally and generally used. Further, ceric oxide has so-called non-Prestonian properties, so that the polishing rate decreases in the case where the polishing pressure is below a certain value. Therefore, it is possible to prevent the silicon oxide film from being polished excessively in the element isolation region at a stage where the polishing stopper film is exposed. As a result, achieved is polishing for high flatness with little dependency on the pattern.
Furthermore, as shown in Japanese Unexamined Patent Publication No. 2006-279050, abrasive materials having high non-Prestonian properties have been proposed in recent years, having a polishing rate with the polishing pressure (pressure for pressing an abrasive material upon polishing) being below a certain value is approximately 20 nm to 50 nm/min. In the case where the surface of a film having protrusions is polished with use of such an abrasive material, polishing has been performed at a polishing rate of approximately 100 nm to 1000 nm/min with existence of protrusions, while the polishing rate suddenly lowers to approximately 50 nm/min or less at a stage where there are no longer protrusions and the surface of the polished film is approximately flat, and thus, polishing barely progresses in comparison with normal polishing rates.
In other words, even in cases where there are no polishing stopper films made of a material different from that for the film to be polished, as in an interlayer insulating film having protrusions formed after semiconductor elements or metal wires are formed, the polishing rate automatically and suddenly lowers (polishing hardly progresses, this state is referred to as “automatically stopping state” hereinafter) at the stage where the protrusions are removed, with use of abrasive grains (abrasive material) having high non-Prestonian properties as described above. Thus, a polishing process can be carried out on such an interlayer film for high-level flatness.
In the case where a flattening process (polishing process) is carried out on a film to be polished having protrusions with use of abrasive grains having high non-Prestonian properties as described above, polishing automatically slows down when the protrusions are removed and the surface of the polished film is substantially flat. Therefore, even when there are defects such as scratches on the surface of the polished film, they are not removed and remain as they are, and thus, there arises a problem that the density of defects on the surface of the polished film is very high even after the flattening process. In addition, the polishing rate is very low after protrusions are removed, and therefore, there arises a problem that it is difficult to control the amount of polishing in the CMP process for each lot to be processed in accordance with the amount of film formed as measured in advance of the CMP process.